Reflective solder mask layer for LED phosphor package

ABSTRACT

A mounting substrate has a patterned metal layer defining a plurality of top metal bond pads for bonding to bottom metal bond pads of LED dies. A solder mask layer is formed over the mounting substrate, where the mask has openings that expose the top metal bond pads and protects metal traces on the substrate. The mask layer is a highly reflective white paint. The exposed top metal bond pads are then wetted with solder. The LED dies&#39; bottom metal bond pads are then soldered to the exposed top metal bond pads, such that the mask layer surrounds each LED die to reflect light. A reflective ring is affixed to the substrate to surround the LED dies. A viscous phosphor material then partially fills the ring and is cured. All downward light by the LED dies and phosphor is reflected upward by the ring and solder mask layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/103,474, filed Jun. 10, 2016, which is a § 371 application ofInternational Application No. PCT/IB2014/066349 filed on Nov. 26, 2014and entitled “REFLECTIVE SOLDER MASK LAYER FOR LED PHOSPHOR PACKAGE,”which claims priority to U.S. Provisional Patent Application No.61/917,421 filed Dec. 18, 2013, which are incorporated by reference asif fully set forth.

FIELD OF INVENTION

This invention relates to packages for phosphor-converted light emittingdiodes (pcLEDs) and, in particular, to a package that increases lightextraction.

BACKGROUND

For high brightness applications, it is common to mount an array of LEDdies on a substrate, where the substrate has metal traces thatinterconnect the LED dies and lead to anode and cathode electrodes forconnection to a power supply. It is common for the LED dies to beGaN-based and emit blue light, where a phosphor (e.g., a YAG phosphor)is deposited over all the LED dies. The combination of the blue lightleaking though the phosphor and the yellow-green phosphor light createswhite light.

Some light from the LED dies and some light from the phosphor is emittedin the downward direction and is partially absorbed by the substrate.Further, if the individual LED dies are also mounted on submounts(typically much larger than the LED die), and the submount electrodesare bonded to the substrate, the submount surface also absorbs some ofthe LED and phosphor light. Such absorption by the substrate andsubmount reduces the overall efficiency of the module.

What is needed is a packaging technique for phosphor-converted LEDs thatresults in more light being emitted by the package.

SUMMARY

In one example of the invention, a starting substrate comprises aluminumfor sinking heat. A thin dielectric layer is formed over the top surfaceof the substrate, and metal traces are patterned over the dielectric.The metal traces define small-area solder pads for a plurality of LEDdies, larger anode and cathode electrodes (also solder pads) for themodule, and interconnections between the LED dies and electrodes.

In one embodiment, screen printing is used to deposit a solder mask overthe substrate. The solder mask deposits a dielectric that has openingsexposing the various electrodes and LED die solder pads. The dielectricis a highly reflective material, such as a binder including TiO₂, ZiO₂,VO₂, or other suitable reflective particles, that scatter and reflectany impinging light. The reflective material may be the same white paintthat is used in integrating spheres, with a reflectivity greater thanabout 94%.

Next, the LED die electrodes are soldered to the exposed solder pads.The solder mask openings may be small enough such that the peripheriesof the LED dies align with or overhang the reflective material, so thatany downward light from the LED dies will be reflected by the reflectivematerial.

A reflective ring, forming vertical walls, is then affixed to thesubstrate to surround the array of LED dies.

A phosphor is then deposited within the ring (the ring is also used as amold) to encapsulate the LED dies and wavelength convert the lightemitted from the LED dies. The phosphor also serves to conduct heat awayfrom the LED dies. In one embodiment, the LED dies emit blue light, andthe blue light plus the phosphor light creates white light.

The LED dies may or may not include a submount. A submount is typicallyused to simplify handling, add mechanical strength, and/or to providerobust and simple electrodes for soldering to a circuit board. If such asubmount is used, the submount is as small as practical so as to notintercept the downward light from the LED semiconductor layers.

Dome lenses may be molded over the LED dies prior to the deposition ofthe phosphor for improving light extraction.

The reflective solder mask adds no additional steps, yet greatlyimproves the efficiency of the LED module.

Other embodiments are described.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a cross-sectional view of a prior art LED die.

FIG. 2 is a cross-sectional view of an LED die mounted on a minimal-areasubmount.

FIG. 3 is a bottom view of an LED die or a submount showing theelectrode pattern.

FIG. 4 is a top down view of an aluminum substrate having a dielectriclayer and a patterned metal layer.

FIG. 5 illustrates the substrate FIG. 4 after deposition of a reflectivesolder mask, exposing the solder pads.

FIG. 6 illustrates the substrate of FIG. 5 after LED dies have beensoldered to the solder pads.

FIG. 7 is a magnified cross-sectional view, along line 7-7 in FIG. 5, ofa portion of the substrate of FIG. 5 showing two LED dies havingoptional lenses.

FIG. 8 is a top down view of the substrate of FIG. 6 after a reflectivering is affixed around the array of LED dies.

FIG. 9 is a magnified cross-sectional view, along line 9-9 in FIG. 8, ofa portion of the substrate of FIG. 8 illustrating the ring being atleast partially filled with an encapsulating phosphor mixture.

Elements that are the same or similar are labeled with the same numeral.

DETAILED DESCRIPTION

FIG. 1 illustrates a conventional LED die 12. Although a flip-chip dieis shown in the examples, the present invention is applicable to anytype of LED die, including vertical LED dies, lateral LED dies, etc.

The LED die 12 includes a bottom anode electrode 14 coupled to a p-layer16 and a bottom cathode electrode 18 coupled to an n-layer 20 by aconductor 22 that fills in an etched opening, covered with a dielectric,in the p-layer 16 and active layer 24. The electrode configuration maybe more complex and include distributed electrodes for better spreadingof current across the LED die 12. The active layer 24 generates lighthaving a peak wavelength. In one example, the peak wavelength is a bluewavelength, and the layers 16, 20, and 24 are GaN-based.

The layers 16, 20, and 24 are epitaxially grown over a substrate 26,such as sapphire. Alternatively, the growth substrate may be removed andreplaced by a transparent support substrate affixed to the semiconductorlayers by an adhesive or by other techniques. Alternatively, there is nosupport substrate, although handling of the thin LED semiconductorlayers becomes more difficult.

As shown in FIG. 2, the LED die 12 may be optionally mounted on asubmount 28 for ease of handling, mechanical support, heat sinking, andsimplifying the electrode structure for mounting on a printed circuitboard. The submount 28 includes a thermally conductive body 30, bottomelectrodes 32 and 34, and vias 36 and 37 connecting the bottomelectrodes 32/34 to the LED die electrodes 14/18. Additional pads (notshown) on the submount surface are used to connect the LED dieelectrodes 14/18 to the vias 36/37. The typical submount is much largerthan the LED die 12 to accomplish the above-mentioned functions.However, in the preferred embodiment of the invention, when using asubmount, it is desirable to make the submount much smaller than thetypical size to minimize light absorption by the submount surface. Inone embodiment, the growth substrate 26 is removed after the LED die 12is mounted on the submount 28.

The term “LED die” hereinafter refers to either the bare chip (e.g.,FIG. 1) or the chip mounted on a submount (e.g., FIG. 2).

FIG. 3 illustrates a possible bottom electrode configuration for eitherthe LED die 12 or the submount 28.

In the remaining figures, it is assumed a submount is not used. However,the LED die/submount may be substituted for the bare LED die 12 in theremaining figures.

FIG. 4 is a top down view of a substrate 40 for an array of LED dies 12.FIG. 7 shows a cross sectional view of the substrate 40. The substrate40 may include a body 41 formed of aluminum or an alloy for heatsinking. The substrate 40 may have any shape, such as rectangular,circular, etc. The length or diameter of the substrate 40 depends on thenumber of LED dies it supports and will typically range from 1 cm to 4cm. The substrate 40 is typically connected in an array of substrates bynarrow connectors that are later cut for singulation to simplifyhandling and speed processing.

A non-conductive dielectric layer 42 (FIG. 7) is formed over the body 41for electrical insulation.

A patterned metal layer, such as copper, is formed over the dielectriclayer 42 to define small solder pad groups 44A, 44B, 44C, and 44D foreach of the LED dies 12 in the array, and to define larger solder pads46 and 48 for the array's anode and cathode electrodes. The solder padgroups 44A-D may include one or more heat sink connections. The metallayer also forms interconnections 50 between the LED dies 12 and theelectrodes 46 and 48. In the example, only four LED dies 12 areconnected in series and parallel. In other embodiments, more or fewerLED dies 12 may be interconnected in series and/or parallel to achievethe desired electrical characteristics and flux.

A problem with the related prior art LED modules is that a significantportion of the LED light emitted downward is absorbed by the mountingsubstrate. The most absorption by the substrate occurs in the vicinityof each LED die 12, since that is where the light is the brightest. Thepresent invention minimizes such absorption.

In FIG. 5, a white (diffusing) paint 52 is screen printed over thesubstrate 40 in all places except where solder is to be applied. FIG. 7is a discontinuous cross-section along line 7-7 in FIG. 5 and shows thepaint 52 in cross-section. The screen is a mesh that is patterned toblock the deposition of the white paint 52 over the solder pad groups44A, 44B, 44C, and 44D. The white paint 52 is a viscous dielectric thatis then cured. Accordingly, the white paint 52 replaces a conventionalsolder mask, and no additional steps are required. A solder maskrestricts any deposited solder, such as molten solder that flows overthe substrate 40, to areas that are exposed by the mask.

Examples of a diffusing white paint include a binder (such as silicone)infused with particles of TiO₂, ZiO₂, VO₂, or other suitable reflectivescattering particles.

In another embodiment, the white paint is applied, in conjunction with amask, by jetting, spraying, a photolithography process with evaporation,or other technique.

Commercially available white paints are greater than 94% reflective forvisible wavelengths and are sometimes used in integrating spheres forlight measurement. The white paint 52 material should be thermallyconductive. Preferably, the reflectivity of the white paint 52 is atleast 90% for visible light.

By applying the white paint 52 as the solder mask, it is ensured thatthe reflective material extends to the edge of the LED dies 12 and evenunder the LED dies 12 between the electrodes. Accordingly, the whitepaint 52 maximizes the reflective surface area of the substrate, limitsthe applied solder to only the areas exposed by the solder mask, andacts as a corrosion barrier for the metal traces, without any addedprocess steps, so there is synergy in the inventive process.

In another embodiment, a conventional solder mask material (e.g., usedfor non-LED circuit boards) is applied directly over the substrate 40followed by the deposition of the white paint 52 (using the same maskpattern) if the conventional solder mask material is desired foradequate protection of the substrate 40 and to reduce costs. In such acase, the white paint 52 protects the solder mask material fromdegrading due to the high intensity blue or UV light.

A solder 54 (FIG. 7) is then applied to the various solder pads forwetting. The solder 54 may be screen-printed, or applied with a soldermask, or applied in any other conventional way. The solder 54 may be asolder paste.

In FIG. 6, the LED die 12 electrodes (or submount electrodes) aresoldered to the substrate's solder pads using a heating process. Asseen, there is no gap between the white paint 52 and the edge of the LEDdies 12. If the LED dies were vertical LED dies, only the bottomelectrode would be directly soldered to one of the substrate's 42 solderpads, and the top electrode would be wire-bonded to another solder pad.If the LED dies were lateral LED dies, the bottom thermal pad would besoldered to one of the substrate's 42 solder pads and both topelectrodes would be wire-bonded to associated solder pads. In virtuallyall LED dies, there is at least one bottom metal bond pad that serves asan electrical and/or thermal pad.

In another embodiment, solder is not used. Instead the bonding may be byan ultrasonic weld, a conductive adhesive (electrically and thermallyconductive), or using other techniques. In such cases, the “solder mask”would be referred to by a suitable name but would still define the areason the substrate 40 where the bottom metal bond pad of the LED die is tobe bonded, and the exposed areas would be approximately the size of theLED die.

FIG. 7 is a compressed and magnified cross-sectional view of a portionof the substrate 40 along line 7-7 in FIG. 5 cutting across two of theLED dies 12 in the horizontal direction. A solder 54 is shown connectingthe LED die electrodes to the solder pad groups 44A and 44D.

Domed lenses 58 may optionally be molded over the LED dies 12 forimproved light extraction and protection of the LED dies 12. In analternative embodiment, LED dies 12 may be enclosed in domed lens priorto soldering, with or without a submount.

In FIG. 8, a reflective ring 60 is affixed to the surface of thesubstrate 40, surrounding the array of LED dies 12. The ring 60 may be areflective metal or a material coated with a reflective layer. The ring60 may be affixed with silicone or epoxy.

FIG. 9 is a compressed and magnified cross-sectional view of a portionof the substrate 40 along line 9-9 in FIG. 8 cutting across two of theLED dies 12 in the horizontal direction and the ring 60, where walls ofthe ring 60 extend above the tops of the LED dies 12 and reflect and mixthe light.

As also shown in FIG. 9, a viscous phosphor mixture 62 is depositedwithin the ring 60, acting as a mold, and cured. The phosphor mixture 62may be a mixture of silicone and phosphor particles. Many ways todeposit the phosphor mixture 62 are envisioned, such as by using asyringe, screen printing, positioning a pre-formed tablet in the ringthat is then melted, etc. The phosphor mixture 62 encapsulates the LEDdies 12 for added protection.

The thickness of the phosphor mixture 62 and phosphor particle densityare controlled to achieve the desired combination of blue light leakageand phosphor conversion. The phosphor may be a single phosphor (e.g.,YAG) or a combination of phosphors (e.g., YAG and red, or green and red,etc.) to achieve the desired color emission.

Preferably, the indices of refraction of the various layers are selectedto provide a transition from the high index GaN to the low index of airto minimize total internal reflection (TIR).

The reflective surface of the substrate 40 under the phosphor mixture 62reflects back over 94% of all the phosphor light emitted in a downwarddirection.

Any number of LED dies 12 may be mounted on the substrate 40, and thediameter of the ring 60 is that needed to surround the LED dies 12 andmay be adjusted accordingly. Any number of the resulting substrates 40can be connected in a system to achieve any desired flux. In oneembodiment, the resulting structure emits white light. Other emissioncolors are possible by selecting different LED dies and phosphors. Aquantum dot material may be substituted for the phosphor mixture 62.

An improvement in efficiency of greater than 10% is typically achievedby using the invention.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications asfall within the true spirit and scope of this invention.

What is claimed is:
 1. A chip on board (COB) light-emitting diode (LED)comprising: a circuit board including: a plurality of conductive pads ona surface of the circuit board, a plurality of interconnects, on thesurface of the circuit board, that electrically couple one or more ofthe conductive pads to one or more power supply pads, and a reflectivesolder mask layer on the circuit board with portions of the plurality ofconductive pads exposed through openings in the reflective solder masklayer; an LED device electrically coupled to one or more of theconductive pads; a dome lens over the LED device, the power supply padsdisposed on the circuit board outside a perimeter of the dome; and awavelength conversion material over the dome lens, such that the domelens separates the wavelength conversion material from the LED device.2. The system of claim 1, wherein the conductive pads form a pluralityof conductive pad groups and each of the conductive pad groups comprisesa first conductive pad and a second conductive pad.
 3. The system ofclaim 2, wherein the portions of the plurality of conductive pad groupsexposed from the reflective solder mask layer include a portion of eachfirst conductive pad and a portion of each second conductive pad.
 4. Thesystem of claim 3, further comprising solder disposed in each of theopenings in the reflective solder mask layer that provide the electricalcoupling between each of the LED devices and the respective first andsecond conductive pads.
 5. The system of claim 2, wherein the powersupply pads are anode and cathode pads for the LED device.
 6. The systemof claim 1, wherein the LED device includes a first conductive pad and asecond conductive pad, and the reflective solder mask layer completelysurrounds the LED and is disposed under the LED between the first andsecond conductive pads.
 7. The system of claim 1, further comprising awavelength converting material over the plurality of LED devices and atleast partially filling a reflective ring surrounding the LED device. 8.The system of claim 1, wherein the reflective solder mask layer isreflective of at least 90% of visible light.
 9. A chip on board (COB)LED comprising: a circuit board; an array of LED devices on a surface ofthe circuit board; a reflective ring on the surface of the circuit boardand surrounding the array of LED devices; a respective dome lensdisposed over each of the plurality of LED devices; power supply padsdisposed on the circuit board outside a perimeter of the dome; areflective solder mask layer situated inside and outside of therespective dome lens of the plurality of LED devices; and a wavelengthconverting material over the LED array, including the dome lenses, andat least partially filling the reflective ring, the dome lens separatingthe LED devices from the wavelength converting material.
 10. The LEDlighting system of claim 9, further comprising: a plurality ofconductive pad groups on a surface of the circuit board; and areflective solder mask layer covering the entire surface of the circuitboard with portions of the plurality of conductive pad groups exposedthrough openings in the reflective solder mask layer.
 11. The LEDlighting system of claim 10, wherein each of the conductive pad groupscomprises a first conductive pad and a second conductive pad.
 12. TheLED lighting system of claim 11, wherein the portions of the pluralityof conductive pad groups exposed from the reflective solder mask layerinclude a portion of each first conductive pad and a portion of eachsecond conductive pad.
 13. The LED lighting system of claim 10, whereinthe reflective solder mask layer is reflective of at least 90% ofvisible light.
 14. A method of manufacturing a chip on board (COB) LEDcomprising: providing a circuit board including a plurality of solderpad groups on a surface thereof; forming a reflective solder mask layerover the surface of the circuit board covering all of the surface exceptregions of the solder pad groups; forming a plurality of LEDs; providinga dome lens over each of the LEDs, the reflective solder mask layersituated inside and outside of the dome lens; disposing power supplypads on the circuit board outside a perimeter of the dome; applyingsolder to the regions of the solder pad groups; disposing the pluralityof LEDs in contact with the solder to electrically and mechanicallycouple one of the plurality of LEDs to each of the solder pad groups;forming a reflective ring on the circuit board and over the solder masklayer; and at least partially filling the reflective ring with awavelength converting material.
 15. The method of claim 14, wherein theforming the reflective solder mask layer comprises screen printing thereflective solder mask using a mesh patterned to block deposition of thereflective solder mask over the solder pad groups.
 16. The method ofclaim 14, wherein the providing the dome lens over each of the LEDscomprises molding the dome lens over each of the LEDs after soldering.